fix(native): show the read verdict in the FileViewerHost body with a Retry button (#2)

After `fetcher.read` returned `{ error }` the host set the top error bar but
`content` stayed `null`, so `loading || html === null` kept the "Fetching …"
spinner drawing under the message and the only way out was close/reopen.

- On a read failure the body now renders the fetcher's verdict (e.g. the
  liveness-aware "This session isn't responding — is it online?") with a
  Retry button that re-runs `fetcher.read` for the same request; while the
  retry is in flight the normal loading state shows again.
- The top error bar is reserved for failures on top of a displayed file
  (save, editor bridge), so the same message is never shown twice.
- The `!fetcher` "No active session" branch was unreachable behind the
  spinner condition; it is checked first now.

Adds the first component spec in @codeam/ide-native: react-dom into jsdom
with thin DOM stubs for the RN primitives (`src/test/setup.tsx`), driven
through the real `FileViewerProvider`.

Refs: codeagent-rtbh

Co-authored-by: Edgar Durand <edgar@privacyhawk.com>

Author: edgar-durand